1. Field of the Invention
The present invention relates to audio signal transmission in general and in particular to the use of a capacitive network placed between the positive and negative conductors of audio signal transmission line and/or an inductive network placed in series or parallel with the positive and/or negative conductor of an audio signal transmission line, to significantly reduce or eliminate audible pre-emphasis created by non-uniform storage of energy within the line.
2. Description of the Relevant Art
An audio signal transmission system usually comprises an audio signal generator and a load coupled to the generator by means of an audio signal transmission line. For example, the generator may comprise an acoustic transducer, such as a microphone, preamplifier or an amplifier; the load may comprise an amplifier or a speaker; and the audio signal transmission line comprises a pair of conductors such as twisted or untwisted, single or multistrand wires or a coaxial cable. One member of the pair, the send line, connects the positive terminal of the generator to the positive terminal of the load and the other, the return line, connects the negative terminal of the generator to the negative terminal of the load.
The audio frequency spectrum is generally considered to encompass a frequency range from 20 Hz to 20 kHz. In transmitting music and other signals having a broad range of frequencies under certain conditions an audible pre-emphasis can be detected in audio signal transmission systems coupled with audio signal transmission cable. For example, in one particular type of audio cable, there is an emphasis of the higher audio frequency components of musical information. Thus, a musical signal containing information of frequencies greater than 100 Hz audibly had more weight and substance than the information at or below 100 Hz through the speakers in the audio signal transmission system. To the human ear, this sounds unnatural, and is therefore undesirable.
Noise at certain frequencies can create the appearance of unwanted emphasis or pre-emphasis of certain sounds at unwanted times, as noise may cause certain desired sounds, particularly at lower frequencies, to become less "focused". The solution to this type of pre-emphasis is to reduce or eliminate noise. In U.S. Pat. No. 4,954,787, a circuit is provided that comprises a discrete capacitor coupled in series with a discrete resistor so as to reduce noise in audio transmission systems.
The apparatus and method described in U.S. Pat. No. 4,954,787 constrains the chosen values of the discrete capacitance and the discrete resistance such that the total capacitance of the audio transmission system, i.e. the sum of the distributed capacitance of the audio transmission line and the discrete capacitance, and the discrete resistance constitute an RC time constant that is above the audio frequency range but within the passband of the audio signal source. This approach necessarily constrains the patented noise suppression network to comprise small values of capacitance and large values of resistance, since it is taught that it is desirable to have a discrete resistor that is equal to or greater than the output impedance of the amplifier so as not to unduly load the amplifier down at high frequencies, and that the value of the discrete capacitance be as low as possible to save space.
In U.S. Pat. No. 5,142,252, a transmission line is provided that comprises a compensating inductance added to the line to overcome the parasitic and dielectric capacitance of the line and reduce noise generated in the line by low frequency, low level audio signals. The necessary magnitude of the inductance depends upon the parasitic and distributed capacitance of the transmission line, and the patent describes creating a low impedance path to low audio frequencies, which inherently suggests the desirability of a low level of energy storage by the inductor.
Noise as such, however, is not the only source of transmission line created pre-emphasis. Therefore, the circuits disclosed by U.S. Pat. Nos. 4,954,787 and 5,142,252 do not eliminate all sources of pre-emphasis in a transmission line. In particular, they do not eliminate the pre-emphasis caused by having significantly differing amounts of energy storage in an audio cable as a function of frequency, which can affect sound quality over the entire range of audio frequency spectrum.